There are copious invisible files on your Mac. Most of them are very important parts of OS X and cannot be deleted. That is probably where much of the space your missing has gone. Also, formatted drives are always much smaller than the hard drive companies tell you. Additionally, if you do a "Get Info" on a folder or file to see the size, go by the number in-between the parenthesis', as it will like be the number the drive manufacturers are using (it's a bigger number). Temporary cache files can sometimes eat up a ton of space as well. Finally, did you calculate the size of the Desktop folder/s for each user on your machine (and the trash)?
Drives are really cheap now, I'd buy a much bigger one if possible (heck get two, they really are a bargain for the 250gb and under ones).